i) Field of the Invention
This invention relates to a novel process for preparing 2-hydroxy-2, 5, 9-tetramethyldecalyl ethanol which is useful as an intermediate in the synthesis of a fragrance.
ii) Description of the Background Art
3a, 6, 6, 9a-Tetramethyl-(3ar, 5at, 9ac, 9bt)-dodecahydro-naphto2 1-b]furan (hereinafter referred to as ambrox) is an odorlferous substance which characterizes the odor of ambergris, and it is indispensable for compounding amber-note fragrances. Conventionally, the following synthesis route via norambrenolide (I) as a synthesis intermediate has been taken as most advantageous, where the starting material is either sclareol obtained from vegetable essential oil or .alpha.-(or.alpha.-) monocyclohomofarnesic acid derived from dihydro .alpha.-(or.alpha.-)ionone: ##STR3##
Reports have been published for the various reaction steps. Among the processes for obtaining compound (II), which is 2-hydroxy-2, 5, 5, 9-tetramethyldecalyl ethanol (hereinafter referred to as diol). by reducing compound (I). which is 3a, 6, 6, 9a-tetramethyl-decahydro-naphto[2, 1-b]furan-2(1H)-one (hereinafter referred to as norambrenolide), the following processes are known to give a high yield:
1) A process which uses lithium aluminum hydride. PA1 2) A process which uses bis-(2-methoxyethoxy)sodium aluminum hydride (Japanese Patent Application (Kokai) No. 239481/1985).
The process 1), however, is neither convenient nor economical because lithium aluminum hydride is expensive, and special care must be taken since the compound is very sensitive to moisture. Additionally, solvents to be used must be purified under strict control. The process 2), on the other hand, uses a significant amount of an expensive reagent, namely, 50-200% by weight of bis-(2-methoxycthoxy)-sodium aluminum hydride based on the total amount of a starting material. Thus this process is not economical.
In the meantime, a process utilizing a mixed solvent of t-butanol and methanol has recently been proposed for reducing lactones with sodium boron hydride [Bull. Chem. Soc. Jpn., 57, 1948-1953(1984)]. Although this process has been attempted for the reduction of norambrenolide, it was found that the process requires sodium boron hydride in an amount of more than 31% by weight, based on the substrate; and a significant amount of solvent, i.e.. more than ten times (1000%) by volume (or seven times (700%) by weight) of the substrate for completing the reaction. Therefore, from the viewpoint of productivity and economy, this process is not suitable for production on an industrial scale.
Accordingly, it has been hoped to develop a new process for reducing norambrenolide with a reagent which is easy to-obtain and inexpensive so as to obtain diol (II) at a high yield.